1. Field of the Invention
The present invention relates generally to electronic circuits, and more specifically to regulated power supplies.
2. Description of the Prior Art
The quality of the power supply voltage is important to the proper operation of many electronic devices. Many types of supplies have been designed to provide, via filtering, supply levels having a constant voltage level and minimal noise. These include passive designs such as LC filters, and active designs such as switching regulated supplies.
Active filters have advantages in that they are smaller and lighter than passive designs incorporating large inductors. In addition, electronic circuitry for building active regulators has become relatively inexpensive, so that active filters can be fabricated to be cost competitive with LC passive designs.
One environment which places harsh demands on electronic circuitry is found in the field of automotive electronics. This is especially true of power supplies, which must regulate an unpredictable and fluctuating battery voltage as well as operate under temperature and humidity extremes. Battery voltages vary widely as a result of battery conditions and other switching loads on the system, such as air conditioners.
Voltage supplies for radios and similar audio electronics are typically regulated at 5 or 10 volts for automotive applications. This is low enough that the battery voltage remains higher than the regulated voltage under most conditions. Battery voltages can reach approximately 14 volts, but the regulated voltage must be lower since such higher battery voltages cannot be counted upon. Typically, the supply for the power amplifier is tied to the battery potential. This means that no regulation takes place for the output stage. Only the internal circuitry of a radio, or other audio device is tied to a regulated supply.
A 10 volt set point for regulation is high enough to provide adequate power to an audio output amplifier under most conditions. However, audio programs sometimes have loud passages that drive the audio output amplifier into saturation and clipping. This causes distortion of the output signal. The distortion due to clipping could be minimized or eliminated in many cases if a higher supply voltage could be used to drive the output amplifier.
It would therefore be desirable to provide a regulated power supply which provides a higher supply voltage in response to increased output signal demands. It would be desirable for such higher voltage to be regulated, and for the supply to provide only a lower regulated voltage when the higher voltages are not required.